Downhole Data at the Speed of Light

Fiber Optics with Apache Oil

A past SPE general meeting featured an insightful presentation on the implementation of fiber optics technology adopted by Apache oil company, in completions and production operations.

What is Fiber Optics?

Fiber optics is an information transmission technology that uses the principle of “total internal reflection” (TIR) to transmit data through glass wires, continuously. These wires are cased with different indices of refraction, which ensures that TIR always occurs. TIR is the process where light (or really any wave) acts incident on a boundary of a surface of different refractive index, at an angle greater than a known critical angle, and is completely reflecting back into the incident surface, with no intensity being lost through refraction. This relationship can be shown as:

Where C is the critical angle, and n is the refractive index of the reflecting surface.

Fiber optic cables can stretch out for hundreds of feet, and its design must ensure that TIR always occurs, and no energy/data is lost to refraction. A basic design for a fiber optic cable consists of a hollow core covered by one (or multiple) layers of glass cladding to protect the fibers within. In the oil and gas industry, fiber optics are cased in multiple hard layers and cemented in the annulus during workovers and well interventions in order to withstand the high temperatures and pressure conditions. The highest priority always lies in ensuring that the sensitive fibers are not damaged.

Fiber Optics and The Oil and Gas Industry:

The usefulness of fiber optics in modern technology cannot be underplayed. By transmitting light, fiber optics can transmit data in the order of terabytes a day, making it the most effective well logging and measurement system available in oil and gas.

The use of fiber optics is an emerging technology, with it’s first use in production logging cited in 1993. Since then, it’s popularity has increased and there have been over 4000 runs to date, all over the world. The reason for this is primarily because the cost has come down over the years, and fiber optics provides more accurate reservoir and completions work than any other model. The technology has also become more reliable over time. The most typical use of this technology is to gather data during various kinds of operations like during/after fracking.

Mechanism Used

Fiber Optics Sample with typical outer casing

Fiber optics typically consist of three components:

Fiber Log and Fiber coil (Retrievable)

Tubing (Permanent)

Casing (Permanent)

In an NCS single stage completions well, we install fiber optics on the outside of the casing. Strong casing is needed to withstand the temperature and pressure conditions. This protection is necessary, because if the cable is damaged, all data is lost. A heel gauge is then installed to get readings at any time.

Another critical component of the apparatus is the interrogator unit, which shoots laser and reflects off the light scattered back to get readings. This unity can record up to 200KHz of light, and has many useful applications such as monitoring well life or creating a vertical seismic profile.

The Sensing itself can occur through one of two methods:

Distributed Acoustic Sensing (DAS)

This measures noise outside the port, on a waterfall plot, which displays fractures and exit points as a function of time.

Distributed Temperature Sensing (DTS)

This measures fluid temperature through another waterfall plot which allows us to interpret temperature through color, i.e. red means hot and blue means cold.

The greatest advantage in using fiber optics is that it provides a more accurate model of operations than tradition temperature and pressure logs, and these can be more sensitive in detecting fracture points by mapping strain as well. Another great benefit is that we can see the time and place the fracture hits on the fiber optic, and this can be used as a calibration point for fracturing models.

Example of a Waterfall Plot

Conclusions

Fiber optics can help with…

Well spacing

Cluster efficiency Not 100% as detected by optics

Simulated Rock Volume (SRV). An added advantage is that known data points can be used as calibrations for new data. We can tell which part of the rock is the most productive

Rock productivity We can tell which part of the rock is the most productive

Fiber optics is a wonderful new technology that is being implemented in our industry and it’s definitely possible to see many other innovations of the same nature in the future as well.